Theses Doctoral

# Quantum Hall transport in graphene and its bilayer

Zhao, Yue

Graphene has generated great interest in the scientific community since its discovery because of the unique chiral nature of its carrier dynamics. In monolayer graphene, the relativistic Dirac spectrum for the carriers results in an unconventional integer quantum Hall effect, with a peculiar Landau Level at zero energy. In bilayer graphene, the Dirac-like quadratic energy spectrum leads to an equally interesting, novel integer quantum Hall effect, with a eight-fold degenerate zero energy Landau level. In this thesis, we present transport studies at high magnetic field on both monolayer and bilayer graphene, with a particular emphasis on the quantum Hall (QH) effect at the charge neutrality point, where both systems exhibit broken symmetry of the degenerate Landau level at zero energy. We also present data on quantum Hall edge transport across the interface of a graphene monolayer and bilayer junction, where peculiar edge state transport is observed. We investigate the quantum Hall effect near the charge neutrality point in bilayer graphene, under high magnetic fields of up to 35~T using electronic transport measurements. In the high field regime, we observe a complete lifting of the eight-fold degeneracy of the zero-energy Landau level, with new quantum Hall states corresponding to filling factors $\nu=0$, 1, 2 and 3. Measurements of the activation energy gap in tilted magnetic fields suggest that the Landau level splitting at the newly formed $\nu=$1, 2 and 3 filling factors does not exhibit low-energy spin flip excitation. These measurements are consistent with the formation of a quantum Hall ferromagnet. In addition, we observed insulating behavior in the two terminal resistance of the $\nu=$0 state at high fields. For monolayer graphene, we report on magneto-resistance measurements at the broken-symmetry of the zero-energy Landau level, using both a conventional two-terminal measurement of suspended graphene devices, which is sensitive to bulk and edge conductance, and a Corbino measurement on high mobility on-substrate devices, which is sensitive to the bulk conductance only. At $\nu=0$, we observe a vanishing conductance with increasing magnetic fields in both cases. By examining the resistance changes of this insulating state with varying perpendicular and in-plane fields, we probe the spin-active components of the excitations in total fields of up to 45 Tesla. Our results strongly suggest that the $\nu=0$ quantum Hall state in single layer graphene is not spin polarized, while a spin-polarized state with spin-flip excitations forms at $\nu=1$. For monolayer and bilayer graphene junction system, we first present a surface potential study across the monolayer/bilayer interface. Then we present experimental investigations of the edge state transition across the interface in the quantum Hall regime. Both monolayer graphene (MG) and bilayer graphene (BG) develop their own Landau levels under high magnetic field. While transport measurements show their distinct quantum Hall effects in the separate parts of the monolayer and bilayer respectively, the transport measurement across the interface exhibits unusual transverse transport behavior. The transverse resistance across the MG/BG interface is asymmetric for opposite sides of the Hall bar, and its polarity can be changed by reversing the magnetic field direction. When the quantum Hall plateaus of MG and BG overlap, quantized resistance appears only on one side of the Hall bar electrode pairs that sit across the junction. These experimental observations can be ascribed to QH edge state transport across the MG/BG interface. We also present sample fabrication details, particularly the efforts to eliminate mobility-limiting factors, including cleaning polymer residue from the electron beam lithography process via thermal annealing and removing/changing the substrate by suspending multi-probe graphene devices.